Am.  Jour.  Pharm.  ) 
Oct.  1, 1873.  S 
Bromide  in  Iodide  of  Potassium  . 
467 
permanent  precipitate  begins  to  form,  and  I  consider  it  highly  prob- 
able that  the  reaction  which  takes  place  is  analogous  to  that  which 
occurs  in  testing  HON  by  what  is  termed  Liebig's  process,  in  which 
a  volumetric  solution  of  AgN03  is  added  to  a  weighed  quantity  of 
HCN ;  and  when  exactly  one-half  the  cyanogen  is  displaced,  and 
permanent  precipitate  of  AgCN  commences  to  form,  a  soluble  double 
salt  is  first  formed  and  shown  by  the  precipitate  dissolving  as  fast  as 
it  appears  until  exactly  half  the  solution  has  been  used  ;  the  HCN  is 
first  made  slightly  alkaline  with  NallO 
AgNOs+2  NaCN=AgCN,NaCN+NaNOs 
and 
AgCN,  NaCN+AgNQ3=:2  AgCN+NaN03 
and  similarly  with  KI  and  Hg  Cl2 
HgCl2+4  KI=(KI)2HgI2+2KCl 
(KI)2Hg2I2+HgCl2=2  HgI2+2  KC1. 
I  prefer  to  make  the  formation  of  a  permanent  precipitate  as  the 
finishing  point  of  the  process  ;  it  is  much  more  sharply  defined  than 
the  point  at  which  the  precipitate  ceases  to  form.  Should  it  be  pre- 
ferred, however,  to  make  use  of  this  latter  (and  it  is  useful  as  a  check 
on  the  first  part  of  the  process)  the  best  way  is  to  take  a  glass  tube 
open  at  both  ends,  about  15  cm.  in  length  and  5  mm.  diameter,  and 
to  tie  over  one  end  a  small  piece  of  filtering  paper;  on  moistening 
this  and  depressing  the  tube  into  the  iodide  solution,  a  few  drops  may 
be  filtered  off  and  transferred  (by  inverting  the  tube)  to  a  porcelain 
slab  and  small  test  glass,  and  a  drop  of  the  mercuric  solution  added ; 
it  will  then  be  instantly  apparent  whether  the  reaction  is  complete  or 
not. 
I  append  the  result  of  my  experiments  with  KI  containing  quan- 
tities of  KBr. 
Percentage  of  KBr  in  1  gm  KI. 
Precip.  commenced. 
Precip.  ceased. 
0 
8-0  c.c. 
16  0  c.c. 
5 
7*6  c.c. 
15-3  c.c. 
10 
7-3  c.c. 
14-6  c.c. 
15 
6-8  c.c. 
13  6  c.c. 
20 
6-4  c.c. 
12-8  c.c. 
25 
6-0  c.c. 
11-9  c.c. 
30 
5-7  c.c. 
11-3  c.c. 
